Characterization of the developmental endothelial locus-1 (Del1) has provided very important insights into the molecular pathways that endothelial cells use to promote angiogenesis. The major isoform contains a signal sequence, three EGF-like repeats with homology to Notch, and two discoiding like domains. It expressed specifically in endothelial cells in the embryo, and is one of the earliest know markers for endothelial cells, with expression rapidly waning after vessel development. Del1 has been shown to inhibit endothelial cell apoptosis through interaction with alphavbeta3 integrin receptors, and in addition has been shown to have potent angiogenic activity in vivo which is mediated though an as yet unknown mechanism. We propose to identify the angiogenic domain of Del1 through mutagenesis assays using the chick chorioallantoic membrane assay, as well as determining the effects of this domain on cell proliferation and apoptosis using several markers. We well also determine the effects of Del1 protein presented to endothelial cells in the matrix or in solution. We will determine if a cell surface receptor other than integrin receptor avb3 interacts with Del1, and if so, we will test likely molecules such as Notch 4 and tek/tie2 for Del1 binding. Other Del1 binding proteins will be cloned using COS cell libraries screened for Del1 binding or by using Del1 as bait in a yeast two hybrid screen of early embryo or endothelial cell libraries.